Low density counterweight for eccentric driven pump

ABSTRACT

An eccentric for balancing the piston of an eccentric driven pump has a counterweight portion with a relatively low density between 0.07 lbm/in 3  and 0.2 lbm/in 3 , preferably less than 0.1 lbm/in 3 .

CROSS-REFERENCE TO RELATED APPLICATION

This claims the benefit of U.S. Provisional Patent Application No. 60/583,346 filed Jun. 28, 2004.

STATEMENT CONCERNING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

FIELD OF THE INVENTION

This invention relates to reciprocating pumps, and specifically to eccentric driven pumps.

BACKGROUND OF THE INVENTION

Eccentric driven pumps such as reciprocating wobble piston, articulated piston, diaphragm and nutating pumps are well known. Referring to FIG. 1, a typical double-ended eccentric driven wobble piston pump is illustrated. The pump 10 has a motor 12 and a pump assembly 14 at each end of the motor 12. A drive shaft 16 of the motor extends into each pump assembly 14 and is shown at the right end of the pump 10 in FIG. 1. An eccentric 18 is fixed to the shaft 16, for example with setscrew 20 engaging a flat on the shaft 16. Prior to assembling the eccentric 18 to the shaft 16, however, a nib portion 22 (FIG. 3) is pressed into the inner race of a bearing 24 and the outer race of the bearing 24 is pressed into or otherwise fixably secured in the bearing bore 26 of connecting rod 28. Piston head 29 of piston 19 is fixed at the top of connecting rod 28 and mounts a seal cup 32, secured by retainer 31, that forms a sliding seal with the cylinder of the pump assembly 14. This allows the piston head 29 to wobble inside the cylinder while at the same time maintaining a seal with the sidewalls of the cylinder.

Referring also to FIG. 2, the bore 17 in eccentric 18 for the shaft 16 is radially offset from the axis of the nib 22. This is to create an eccentricity to obtain the required motion of the wobble piston 19. The radial offset of the nib 22 from the shaft 16 however creates a relatively complicated balance problem, because as the piston head is reciprocating, the lower portion of the connecting rod is orbiting, with a continuum of imbalance from reciprocation to orbiting in between. This is analyzed, in one way, from the standpoint of the center of gravity of the piston and the instantaneous angle of travel of the center of gravity with respect to the shaft 16 axis. To solve the balance problem in large part, the eccentric 18 is provided with an imbalance portion 30 (FIG. 3) which is designed to offset the imbalance of the nib 22 and piston 19 (the piston 19 includes the connecting rod 28, the seal 32, the retainer 31 and the fastener that secures the retainer).

The counterweight portion 30 in the prior art eccentric 18 is varied in thickness to add or subtract mass, depending on the mass of the piston 19, which is dependent on the size of the pump. As illustrated in FIG. 3, the counterweight portion 30 is of a certain thickness T which for a larger piston or larger eccentricity (stroke) could be increased to the thickness M.

In the prior art, eccentrics 18, and particularly the counterweights portions if the eccentric was a composite, have been made of highly dense materials such as powder metal (e.g., steel), zinc and brass for their strength and their relatively high mass per unit volume. The manufacturing methods required for these materials are relatively high in cost, and the materials themselves are, compared to many other materials, difficult to machine and result in machine tool wear. Therefore, an improved eccentric is needed.

SUMMARY OF THE INVENTION

The invention provides an eccentric of the type described having a counterweight portion of a relatively low density material. The material is relatively inexpensive and easy to machine, and also has the benefit of being able to increase the thickness of the counterweight portion of the eccentric, which reduces a moment that is created by the axial spacing of the piston and eccentric.

The foregoing and other objects and advantages of the invention will appear in the detailed description which follows. In the description, reference is made to the accompanying drawings which illustrate a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partially exploded perspective view of a prior art wobble piston pump construction;

FIG. 2 is an end view of the wobble piston pump of FIG. 1 with the fan removed;

FIG. 3 is a perspective view of a prior art eccentric for the pump of FIGS. 1 and 2 which is made of a relatively highly dense material; and

FIG. 4 is a perspective view of an eccentric of the invention to replace the eccentric of FIG. 3 for the pump of FIGS. 1 and 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 4 illustrates an eccentric 18′ of the invention to replace the eccentric 18 of FIG. 3. In other words, eccentric 18′ is for a pump having a piston of the same mass with the center of gravity of the piston in the same position relative to the shaft as the pump that the eccentric 18 is used with. In shape and features, the eccentric 18′ is similar to the eccentric 18, being made in one integral piece, having a nib 22′, a set screw 20′ and a counterweight portion 30′. However, whereas in the prior art the eccentric 18 would have been made out of highly dense material such as steel, zinc or brass, the eccentric 18′ is made of a material of a lower density than the eccentric 18. The density of the prior eccentric 18 and therefore of its counterweight portion 30 in general was greater than 0.2 lbm/in³. For example, die cast zinc has a density of 0.227 lbm/in³, powder metal steel has a density of 0.247 lbm/in³ and brass has a density of 0.310 lbm/in³. In the eccentric 18′, the density of the eccentric portion 30′ is preferably less than 0.2 lbm/in³, and preferably less than 0.1 lbm/in³. Since the whole eccentric 18′ is made in one monolithic piece of the same homogeneous material, the density of the whole eccentric is also preferably less than 0.2 lbm/in³. For example, aluminum alloy (A380 die cast), which is the preferred material for the eccentric 18′ including the counterweight portion 30′, has a density of 0.098 lbm/in³. This material also has the benefits that it can be die cast, which is a relatively inexpensive manufacturing process, and that it can be easily machined with little machine tool wear.

The lower density material of the eccentric 18′ including counterweight portion 30′ permits increasing the thickness T to being closer or equal to the thickness M. This moves the center of gravity of the eccentric 18′ axially closer to the axial position of the center of gravity of the piston 19, which reduces the shaking moment (about an axis perpendicular to the axis of the shaft 16) that is produced between the eccentric 18′ and the piston when the shaft 16 is rotated. This results in less vibration of the entire pump 10.

The material of the eccentric 18′, including the nib 22′ and counterweight portion 30′, while of low density, must still be sufficiently strong to bear the forces imparted to it, particularly in the area of the nib 22′. Materials with densities below about 0.07 lbm/in³ in general would not exhibit the requisite strength and therefore in general would not be good choices for the eccentric 18′ in general. Thus, the density of the counterweight 30′ should be between 0.07 lbm/in³ and 0.2 lbm/in³, and preferably between 0.07 lbm/in³ and 0.1 lbm/in³. Preferably the whole eccentric is monolithic and homogeneous, made of the same material,

A preferred embodiment of the invention has been shown and described in considerable detail. Many modifications and variations to the preferred embodiment described will be apparent to those skilled in the art. Therefore, the invention should not be limited to the preferred embodiment described, but should be defined by the claims which follow. 

1. In an eccentric driven pump that has a rotary drive shaft that defines an axial direction, an eccentric having a bore in rotary driven engagement with the drive shaft, the eccentric having a circular nib portion with an axis that is offset from the axis of the drive shaft in a radial direction and a counterweight portion that is spaced radially from the axis of the drive shaft, the pump having a wobble piston that is journaled on the nib of the eccentric to be driven in reciprocation by rotation of the drive shaft, the improvement wherein the counterweight portion of the eccentric has a density of less than 0.2 lbm/in³.
 2. The improvement of claim 1, wherein the counterweight portion has a density of less than 0.1 lbm/in³.
 3. The improvement of claim 1, wherein the counterweight portion has a density between 0.2 lbm/in³ and 0.07 lbm/in³.
 4. The improvement of claim 1, wherein the counterweight portion has a density between 0.1 lbm/in³ and 0.07 lbm/in³.
 5. The improvement of claim 1, wherein the nib is made of the same material as the counterweight portion. 